Drona has been in charge now for two days, and so far he has failed in his mission to capture Yuddistira. This despite the fact that they have sacrificed a whole nation of warriors just to keep Arjun away long enough for them to grab his brother. The Trigartas are nearly all dead so they need to come up with a new idea. This time, Drona implements a fancy new formation, based on a wheel, which few know how to break.
It turns out that Abhimanyu is one of those who know its secrets. But with Arjun busy, he is the only one. Helpless to protect himself and his followers any other way, Yuddistira sends the boy wonder into the jaws of death. Even then, they could have protected the boy, except they were stopped by old Jayadratha of Sindh.
Remember this guy? He's actually related in marriage to Duryodhana-- he married Duryodhana's sister. But one wife wasn't good enough for him, and he stupidly tried to run off with Draupadi, without first defeating the Pandavas. He paid the price of humiliation for his impudence: he was beaten severely and had his head shaved by an arrow head by Bhimasena. Too humiliated to return home, Jayadratha remained in the woods, praying to Shiva. When he finally coaxed Shiva to grant him a wish, he asked for the ability to kill all five Pandavas!
This was a bit too much even for Shiva. The best he could do was grant him the ability to stop the Pandavas briefly, but not Arjun. Obviously, now was a perfect moment to use this boon, so Jayadratha rode out to forstall the Pandavas, while Drona, Krpa, Karna & Ashvatthaman ganged up on young Abhimanyu and killed him in an ambush. It was Dushasan's son, Daushasani, who dealt the death blow.
What is Arjun going to say when he finds out they let his son get killed?!?